.\" $OpenBSD: __get_tcb.2,v 1.4 2012/04/12 14:25:57 deraadt Exp $
.\"
.\" Copyright (c) 2011 Philip Guenther <guenther@openbsd.org>
.\"
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.Dd $Mdocdate: April 12 2012 $
.Dt __GET_TCB 2
.Os
.Sh NAME
.Nm __get_tcb ,
.Nm __set_tcb
.Nd get and set the address of the thread control block of the current thread
.Sh SYNOPSIS
.Ft void *
.Fn __get_tcb "void"
.Ft void
.Fn __set_tcb "void *"
.Sh DESCRIPTION
The
.Fn __get_tcb
and
.Fn __set_tcb
functions are for use by librthread and other parts of the system
runtime to retrieve and set the address of the current thread's
thread control block (TCB).
This is used to locate per-thread data such as
.Va errno .
Each kernel-level thread in a process has a separate value for this
address, which can be obtained and changed via these system calls.
New threads (including the first thread of a new process) created using
.Xr fork 2 ,
.Xr vfork 2 ,
or
.Xr __tfork 2 ,
inherit the TCB address of the thread that created them.
.Xr execve 2
resets it to zero.
.Pp
On some platforms, this address is also directly mapped to a CPU
register which can be accessed from userspace.
.Sh RETURN VALUES
.Fn __get_tcb
returns the address of the thread control block of the current thread.
.Sh SEE ALSO
.Xr __tfork 2
.Sh HISTORY
The
.Fn __get_tcb
and
.Fn __set_tcb
system calls appeared in
.Ox 5.1 .
